Electrolyte and lithium ion battery

Abstract

The invention provides an electrolyte and a lithium ion battery. The electrolyte comprises a first additive and a second additive, the first additive has a molecular structure as shown in a formula I: # imgabs0 # formula I; the second additive has a molecular structure as shown in a formula II: # imgabs 1 # formula II; according to the electrolyte provided by the embodiment of the invention, the first additive and the second additive are introduced, so that the cycle performance and the quick charge performance of the battery can be improved under the condition of low liquid injection amount.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and in particular to an electrolyte and a lithium-ion battery. Background Art

[0002] With the rapid popularization of portable electronic devices, new energy electric vehicles and energy storage systems, the market has put forward higher requirements on the performance of secondary batteries: higher energy density, excellent fast charging performance, longer cycle life and higher safety.

[0003] In related technologies, to meet the performance requirements of secondary batteries, the energy density of secondary batteries can be increased by increasing the charging voltage or increasing the capacity of active materials. However, when the specific capacity of active materials is increased, it may lead to a higher coating density and a greater compaction density, further compressing the internal gaps of the battery cell, resulting in a lower electrolyte injection volume. Traditional electrolytes have a lower liquid retention capacity when the injection volume is low. When the liquid retention capacity is low, the fast charging and cycle performance of the battery will deteriorate, and the phenomenon of diving often occurs in the late stage of the cycle. The requirement for fast charging further exacerbates the risk of diving. Summary of the Invention

[0004] The embodiments of the present invention provide an electrolyte and a lithium-ion battery, which can improve the technical problem that the electrolyte cannot maintain the excellent cycle performance and fast charging performance of the battery when the liquid retention volume is low.

[0005] In a first aspect, an embodiment of the present invention provides an electrolyte, comprising: A first additive and a second additive, wherein the first additive has a molecular structure as shown in Formula I: Formula I; The second additive has a molecular structure as shown in Formula II: Formula II; in, R1, R2 and R3 are each independently selected from halogen atoms, C1-C 10 Chain alkyl, C3-C 10 Any one of the cycloalkyl groups; R4 is independently selected from any one of C2-C5 alkenyl and C2-C5 alkynyl; Dashed lines represent conjugated π bonds.

[0006] Alternatively, in one embodiment, the second additive is selected from at least one of the following compounds: Formula II-1; Formula II-2; Formula II-3; Formula II-4.

[0007] Optionally, in one embodiment, the mass ratio of the first additive to the second additive is 1:(1-4).

[0008] Optionally, in one embodiment, taking the total mass of the electrolyte as a reference, The mass percentage of the first additive is greater than or equal to 0.1% and less than or equal to 4%; and / or The mass percentage of the second additive is greater than or equal to 0.1% and less than or equal to 4%.

[0009] Optionally, in one embodiment, the mass percentage of the first additive is greater than or equal to 0.2% and less than or equal to 2%; and / or The mass percentage of the second additive is greater than or equal to 0.2% and less than or equal to 2%.

[0010] Optionally, in one embodiment, the electrolyte injection coefficient is 2.9 g / Ah to 3.2 g / Ah.

[0011] Optionally, in one embodiment, the electrolyte further includes a lithium salt, and the lithium salt includes at least one of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, lithium difluorooxalatoborate, lithium bis(trifluoromethylsulfonyl)imide, and lithium bis(oxalatoborate).

[0012] Optionally, in one embodiment, based on the total mass of the electrolyte, the mass percentage of the lithium salt is 10% to 18%.

[0013] Optionally, in one embodiment, the electrolyte further includes a non-aqueous solvent, and the non-aqueous solvent includes at least one of ethylene carbonate, propylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methylpropyl carbonate, methyl formate, ethyl acetate, methyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, propyl propionate, butyl propionate, methyl butyrate, ethyl butyrate, and ethylene glycol dimethyl ether.

[0014] In a second aspect, an embodiment of the present invention provides a lithium-ion battery, the lithium-ion battery comprising a positive electrode sheet, a negative electrode sheet, a separator and the electrolyte as described above. The positive electrode sheet comprises a positive electrode active material; the positive electrode active material comprises LiCoO2, LiMn2O4, LiMnO2, Li2MnO4, LiFePO4, Li 1+a Mn 1-x M x O2、LiCo 1-x M xO2、LiFe 1-x M x PO4 and Li2Mn 1-x M x At least one of O4; wherein, M is selected from at least one of Ni, Co, Mn, Al, Cr, Mg, Zr, Mo, V and Ti, 0≤a<0.2, 0≤x<1.

[0015] Beneficial effects of the embodiments of the present invention: In an embodiment of the present invention, the electrolyte includes a first additive and a second additive, and the second additive can be preferentially decomposed before the solvent of the electrolyte is oxidatively decomposed to form a stable positive electrode-electrolyte interface film; the second additive is an ionic liquid additive containing multiple functional groups, and its introduction can significantly increase the ion migration number of the electrolyte, so that the electrolyte has a higher conductivity, which helps to improve the ion transmission rate of the electrolyte and enhance the fast charging performance of the battery cell. However, under low liquid retention conditions, the electrolyte is not fully filled in the electrode pores and the diaphragm, which will cause the molecular diffusion path of the second additive in the electrolyte to become longer, and because the second additive has a large molecular weight, it cannot be diffused into the entire electrode in a timely and uniform manner, resulting in a high concentration of additives on the electrode surface and uneven interface reaction. Especially at high rate charge and discharge, the interface reaction rate is fast, the local additives are quickly consumed, and new additives cannot be replenished in time, resulting in a rapid deterioration of the interface stability in this area. The first additive has strong diffusivity and is easily decomposed into highly electronegative sulfate groups when forming a film at the positive or negative electrode. The sulfate groups can attack the caged anion groups in the second additive, causing their phosphorus-oxygen bonds to break. The two interact to form a uniform and stable small molecule structure, which can diffuse evenly throughout the entire electrode, solving the problem of uneven distribution caused by limited electrolyte transmission. In addition, the decomposition products of the first and second additives include inorganic salt components such as lithium alkyl sulfonate, lithium phosphate and LiF. The inorganic salt components can improve the stability of the electrolyte and electrode interface under low liquid retention coefficients. DETAILED DESCRIPTION

[0016] The technical solutions of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the embodiments described are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without making creative work are within the scope of protection of the present invention. In addition, it should be understood that the specific embodiments described herein are only used to illustrate and explain the present invention and are not used to limit the present invention.

[0017] In the related art, in order to meet the performance requirements of secondary batteries, the energy density of secondary batteries can be improved by increasing the charging voltage or increasing the capacity of active materials. However, when the specific capacity of the active material is increased, it may lead to a higher coating density and a greater compaction density, further compressing the internal voids of the battery cell, resulting in a lower electrolyte injection volume. When the injection volume of the traditional electrolyte is low, its liquid retention volume is also low. When the electrolyte is at a low liquid retention volume, the cycle performance of the battery may decline, especially in the late stage of the cycle, the phenomenon of frequent diving occurs; moreover, when the battery is charged and discharged at a high rate, it will accelerate the decomposition and consumption of the electrolyte, aggravate the interfacial side reactions of the battery cell, and lead to poor interface stability in the local area of the electrode, thereby affecting the fast charging performance of the battery.

[0018] In view of this, the embodiments of the present application provide an electrolyte and a lithium-ion battery, aiming to improve the problem that the electrolyte cannot maintain excellent cycle performance and fast charging performance under low liquid retention conditions.

[0019] According to a first aspect of an embodiment of the present application, an electrolyte is provided, comprising: a first additive and a second additive, The first additive has a molecular structure as shown in Formula I: Formula I; The second additive has a molecular structure as shown in Formula II: Formula II; in, R1, R2 and R3 are each independently selected from halogen atoms, C1-C 10 Chain alkyl, C3-C 10 Any one of the cycloalkyl groups; R4 is independently selected from any one of C2-C5 alkenyl and C2-C5 alkynyl; Dashed lines represent conjugated π bonds.

[0020] The electrolyte of the embodiment of the present application includes a first additive and a second additive, wherein the second additive can be preferentially decomposed before the solvent of the electrolyte is oxidatively decomposed to form a stable positive electrode-electrolyte interface film; the second additive is an ionic liquid additive containing multiple functional groups, and its introduction can significantly increase the ion migration number of the electrolyte, so that the electrolyte has a higher conductivity, which helps to improve the ion transmission rate of the electrolyte and enhance the fast charging performance of the battery cell. However, under low liquid retention conditions, the electrolyte is not fully filled in the electrode pores and the diaphragm, which will cause the molecular diffusion path of the second additive in the electrolyte to become longer, and because the second additive has a large molecular weight, it cannot be diffused into the entire electrode in a timely and uniform manner, resulting in a high concentration of additives on the electrode surface and uneven interface reaction. Especially at high rate charge and discharge, the interface reaction rate is fast, the local additives are quickly consumed, and the new additives cannot be replenished in time, resulting in a rapid deterioration of the interface stability in this area.

[0021] In the embodiment of the present application, the first additive is introduced into the second additive. Due to its strong diffusivity, it is easy to decompose into a highly electronegative sulfate group when forming a film at the positive or negative electrode. The sulfate group can attack the caged anion group in the second additive, causing its phosphorus-oxygen bond to break. The two interact to form a uniform and stable small molecule structure, which can diffuse evenly throughout the entire electrode, solving the problem of uneven distribution caused by limited electrolyte transmission. In addition, the decomposition products of the first additive and the second additive include inorganic salt components such as lithium alkyl sulfonate, lithium phosphate and LiF. The inorganic salt components can improve the stability of the electrolyte and electrode interface under low liquid retention coefficients.

[0022] It should also be noted that the sulfate groups in the first additive molecules are easily hydrolyzed, especially in the presence of trace amounts of water, and will decompose to produce sulfuric acidic substances (such as H2SO4). Commercial electrolyte lithium salts (such as LiPF6) are easily hydrolyzed to produce HF (hydrofluoric acid) and PF5. The ring-opening decomposition products of the first additive during storage may catalyze this process, further increasing the acidity of the electrolyte. The embodiments of this application introduce a second additive, whose cage-like conjugated ring structure helps further react with the sulfate groups of the first additive to undergo ring-opening polymerization, thus avoiding the problem of increased acidity caused by the addition of the first additive during storage. The interaction between the two improves the thermal and chemical stability of the electrolyte, forming a high-conductivity and stable electrolyte.

[0023] It should also be noted that the electrolyte of the embodiment of the present application is added with a first additive compound of formula I and a second additive compound of formula II. The compound of formula I will undergo ring opening at the low-potential negative electrode, which will help to copolymerize with the compound of formula II to form a film at the negative electrode, thereby forming a dense protective film on the surface of the negative electrode, effectively inhibiting the decomposition of the solvent and lithium salt in the electrolyte; moreover, the compound of formula II can be oxidized and decomposed in advance before the solvent to form a stable positive electrode-electrolyte interface film, and the two can form a skeleton of a polymer double-layer interface film, thereby improving the stability of the interface film; the skeleton of the double-layer interface film can avoid continuous contact between the electrode material and the electrolyte, prevent the decomposition of the electrolyte, and reduce the damage to the electrode material structure.

[0024] It should be further explained that when the electrolyte uses a compound such as Formula I and a compound such as Formula II, the two compounds together form a double-layer interface film with high conductivity and high thermal stability at the positive electrode interface, effectively reducing the interfacial side reactions of the battery electrolyte and making the electrolyte in a low-consumption state; it helps to improve the interface stability between the electrolyte and the electrode, ensure a high-conductivity and stable electrolyte system, and further improve the battery fast charging performance.

[0025] Therefore, the electrolyte of the embodiment of the present application can enable the battery to have excellent cycle performance and fast charging performance at a low liquid retention coefficient.

[0026] In some embodiments of the present application, the second additive is selected from at least one of the following compounds: Formula II-1; Formula II-2; Formula II-3; Formula II-4.

[0027] By adopting the above scheme, Formula II-2 has a denser electron cloud than Formula II-1, resulting in a stronger interaction with lithium ions, which may affect the ion migration rate. Therefore, Formula II-2 has lower reactivity than Formula II-1. In addition, the stronger rigidity of the alkynyl group can further improve the density of the film, affecting the fast charging performance of the battery.

[0028] Compared with Formula II-1, R1, R2 and R3 of the amino cation part are selected from C5 cycloalkyl groups. The C5 cycloalkyl group has a certain steric hindrance, which is beneficial to improving the structural stability of the amino cation and enhancing the thermal stability of the electrolyte; moreover, R1, R2 and R3 are selected from C5 cycloalkyl groups and can also change the electron cloud distribution around the amino cation. The denser the electron cloud distribution, the more it affects the conductivity of lithium ions, thereby reducing the cycle performance and fast charging performance of the battery.

[0029] It should be noted that the CAS number of formula II-1 is 2681338-34-9.

[0030] In some embodiments of the present application, the mass ratio of the first additive to the second additive is 1:(1-4). For example, the mass ratio of the first additive to the second additive can be 1:1, 1:2, 1:3, 1:4, or any other mass ratio in the range between two adjacent mass ratios.

[0031] By adopting the above scheme, the first additive and the second additive are compounded in a suitable mass ratio, which helps to further improve the battery's excellent cycle performance and fast charging performance at a low liquid retention coefficient.

[0032] In some embodiments of the present application, based on the total mass of the electrolyte, the mass percentage of the first additive is greater than or equal to 0.1% and less than or equal to 4%. Preferably, the mass percentage of the first additive is greater than or equal to 0.1% and less than or equal to 2%. For example, the mass percentage of the first additive can be 0.1%, 0.3%, 0.5%, 0.8%, 1%, 1.2%, 1.4%, 1.6%, 1.8%, 2%, and any value between two adjacent values.

[0033] By adopting the above scheme, the first additive with an appropriate mass percentage provides an appropriate amount of sulfate groups, which helps to attack the caged anion groups of the second additive, thereby forming a stable small molecule structure. This small molecule structure helps to diffuse evenly throughout the entire electrode, which is beneficial to solving the problem of uneven interface stability caused by uneven distribution of the second additive in the electrode; at the same time, it can also solve the problem of the longer molecular diffusion path of the second additive.

[0034] In some embodiments of the present application, based on the total mass of the electrolyte, the mass percentage of the second additive is greater than or equal to 0.1% and less than or equal to 4%. Furthermore, the mass percentage of the second additive is greater than or equal to 0.1% and less than or equal to 2%. Exemplarily, the mass percentage of the second additive is 0.1%, 0.3%, 0.5%, 0.8%, 1%, 1.2%, 1.4%, 1.6%, 1.8%, 2%, and any value between two adjacent values.

[0035] By adopting the above scheme, the second additive with an appropriate mass percentage can help reduce the problem of increased acidity caused by the introduction of the first additive; moreover, the second additive is oxidized and decomposed in advance before the solvent, forming a stable positive electrode-electrolyte interface film. At the same time, the second additive can also copolymerize with the first additive to form a film, together forming the skeleton of the double-layer interface film, thereby improving the stability of the interface film, avoiding continuous contact between the electrode material and the electrode liquid, effectively preventing the decomposition of the electrolyte while reducing the damage to the electrode material structure.

[0036] In some embodiments of the present application, the electrolyte may further include a lithium salt, and the lithium salt may include at least one of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, lithium difluorooxalatoborate, lithium bis(trifluoromethylsulfonyl)imide, and lithium bis(oxalatoborate).

[0037] In some embodiments of the present application, the weight percentage of the lithium salt is 10% to 18% based on the total weight of the electrolyte. For example, the weight percentage of the lithium salt is 10%, 10.5%, 11%, 11.5%, 12%, 12.5%, 13%, 13.5%, 14%, 14.5%, 15%, 15.5%, 16%, 16.5%, 17%, 17.5%, 18%, and any value between two adjacent values.

[0038] By adopting the above solution, the lithium salt meets the above conditions, which helps the ions in the electrolyte to migrate efficiently and stably, thereby improving the battery's rate performance and cycle performance. In addition, it helps maintain the chemical stability of the electrolyte, reduces side reactions during the charge and discharge process, and helps extend the battery's cycle life and improve its safety.

[0039] In some embodiments of the present application, the electrolyte may also include a non-aqueous solvent, and the non-aqueous solvent may include at least one of ethylene carbonate, propylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methylpropyl carbonate, methyl formate, ethyl acetate, methyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, propyl propionate, butyl propionate, methyl butyrate, ethyl butyrate, ether, ethylene glycol dimethyl ether and so on.

[0040] It should be noted that the non-aqueous solvent may include at least one of ethyl acetate, dimethyl carbonate, ethyl methyl carbonate and diethyl carbonate.

[0041] In some embodiments of the present application, the mass percentage of the non-aqueous solvent is 74% to 90% based on the total mass of the electrolyte. Exemplarily, the mass percentage of the non-aqueous solvent is 74%, 78%, 82%, 86%, 88%, 89%, 90%, and any value between two adjacent values.

[0042] By adopting the above solution, the non-aqueous solvent meets the aforementioned conditions, enabling efficient migration and uniform distribution of lithium ions in the electrolyte, thereby improving the battery's charge-discharge efficiency and cycle stability. Furthermore, it helps form a stable SEI film, reducing electrolyte decomposition. Furthermore, it optimizes the battery's thermal stability and reduces side reactions at high temperatures, thereby enhancing battery performance.

[0043] According to a second aspect of an embodiment of the present application, a lithium-ion battery is provided. The lithium-ion battery includes a positive electrode sheet, a negative electrode sheet, a separator, and the electrolyte as described above.

[0044] By adopting the above solution, the lithium-ion battery has all the characteristics and advantages of the aforementioned electrolyte, which will not be repeated here. In general, it has at least the advantages of maintaining excellent battery cycle performance and fast charging performance under low electrolyte retention.

[0045] In some embodiments of the present application, the positive electrode sheet includes a positive electrode active material selected from LiCoO2, LiMn2O4, LiMnO2, Li2MnO4, LiFePO4, Li 1+a Mn 1-x M x O2、LiCo 1-x M x O2、LiFe 1-x M x PO4 and Li2Mn 1- x M x At least one of O4; M is selected from at least one of Ni, Co, Mn, Al, Cr, Mg, Zr, Mo, V, Ti, B and F, 0≤a<0.2, 0≤x<1.

[0046] Illustratively, 0≤a≤0.19, 0.05≤a≤0.15, 0.08≤a≤0.13, 0.1≤a≤0.12; 0≤x≤0.9, 0.1≤x≤0.8, 0.2≤x≤0.7, 0.3≤x≤0.6, 0.4≤x≤0.5, etc.

[0047] It is understandable that Li 1+a Mn 1-x M x O2、LiCo 1-x M x O2、LiFe 1-x M x PO4 and Li2Mn 1-x M xThe selection of M in each chemical formula of O4 is independent of each other and does not affect each other. They can be the same or different. Similarly, in the above-mentioned list of positive electrode active materials, the selection of a and x is also independent of each other and does not affect each other. They can be the same or different.

[0048] The present invention is described in detail below by means of specific examples, which are only some examples of the present invention and are not intended to limit the present invention. The raw materials used in the following examples, unless otherwise specified, are all commercially available products.

[0049] Example 1 1. Preparation of positive electrode sheet The positive electrode active material lithium iron phosphate, the conductive agent carbon black and carbon nanotubes, and the binder polyvinylidene fluoride (PVDF) are added to N-methylpyrrolidone (NMP) in a mass ratio of 94.5:4:1.5 to prepare a positive electrode slurry; the positive electrode slurry is coated on the upper and lower surfaces of the aluminum foil and dried and then cold pressed, and then trimmed, cut and striped to make a positive electrode sheet. The thickness of the single-sided positive electrode material layer is 72μm.

[0050] 2. Preparation of negative electrode sheet Graphite, conductive agent carbon black, binder styrene-butadiene rubber (SBR), and thickener (CMC) are mixed in deionized water in a mass ratio of 94.5:2:2:1.5 to prepare a negative electrode slurry; the negative electrode slurry is coated on the upper and lower surfaces of copper foil and dried, and then cold pressed, trimmed, cut into pieces, and striped to make negative electrode sheets. The thickness of the single-sided negative electrode material layer is 110 μm.

[0051] 3. Preparation of electrolyte In an argon-filled glove box (moisture <10ppm, oxygen <1ppm), ethylene carbonate (EC), ethyl methyl carbonate (EMC) and ethyl acetate (EA) were mixed in a mass ratio of 3:3:4, and fully dried lithium hexafluorophosphate (LiPF6) (10%) and lithium bis(fluorosulfonyl)imide (LiFSI) (4.6%) were added to the mixed solvent. After mixing, lithium salt, a first additive, a second additive and other additives were added according to the mass percentage of each component, and the mixture was mixed again to obtain an electrolyte; vinylene carbonate was used as the other additive, and the mass percentage of vinylene carbonate in the electrolyte was 3%.

[0052] 4. Diaphragm A 10 μm polyethylene film was used as a separator.

[0053] 5. Preparation of batteries The positive electrode sheet, separator, and negative electrode sheet are stacked in order, with the separator placed in the middle of the positive and negative electrode sheets to isolate the positive and negative electrodes. The bare battery cell is wound and the tabs are welded. The bare battery cell is placed in the outer packaging, and the electrolyte prepared above is injected into the dried battery cell. The battery is packaged, allowed to stand, formed, and shaped to complete the preparation of the battery.

[0054] It can be understood that in the electrolyte, between different embodiments, when the total amount of additives (the embodiments of the present application include the first additive, the second additive and other additives) changes, the solvent content also changes accordingly. For example, if the total amount of additives increases by 1%, the corresponding solvent content will decrease by 1%, but the ratio of EC, EMC, and EA in the solvent will still be 3:3:4.

[0055] The preparation methods of the batteries of Examples 2-25 and Comparative Examples 1-5 are the same as that of Example 1, except that the composition of the additives in the electrolyte is different, as shown in Table 1.

[0056] Table 1

[0057] Performance testing: (1) Normal temperature cycle performance: At 25°C, charge to 3.65V with 2C constant current and constant voltage, let it stand for 5 minutes, and then discharge to 2.5V with 1C constant current. Calculate the capacity retention rate after 1500 cycles. Capacity retention rate (%) = (1500th discharge capacity / 1st discharge capacity) × 100%; (2) Liquid retention efficiency: The secondary battery after the room temperature cycle performance test was subjected to an electrolyte retention test. After opening the secondary battery, 3g of dichloromethane was injected into the injection port. The secondary battery was placed in an ultrasonic machine and sonicated for 60 minutes to evenly mix the dichloromethane and electrolyte. The battery was sealed and left to stand at room temperature for one day. The battery was then disassembled in a glove box with a moisture content of less than 0.1ppm and an oxygen content of less than 0.1ppm. The electrolyte was removed and the mass percentage of dichloromethane in the electrolyte was measured using gas chromatography-mass spectrometry (GC-MS) / ion chromatography. The mass of the remaining electrolyte was calculated using the injected dichloromethane amount and the mass percentage of dichloromethane in the electrolyte. The mass of the remaining electrolyte is: ; The electrolyte retention efficiency (%) after 1500 cycles at 2C / 1C at room temperature is: ; The injection volume is the amount of electrolyte injected during the preparation of the secondary battery.

[0058] (3) Fast charging performance: Place the battery in a 25°C environment, charge it to the rated voltage at a constant current of 1C, charge it to a cutoff current of 0.05C at a constant voltage, leave it for 5 minutes, then discharge it to 2.5V at a constant current of 1C, and record the charging capacity as C charge. After charging to the rated voltage at a constant rate, charge it at a constant voltage until the current drops to 0.05C, leave it for 5 minutes, then discharge it to 2.5V at a constant current of 1C, and leave it for 5 minutes. This is a charge and discharge cycle. Repeat the charge and discharge steps for 3 weeks, and perform charge tests at rates of 0.5C, 1C, 2C, 3C, and 4C on the battery in turn. The constant current charging capacity of the last cycle of the 4C rate is recorded as C4, and the 4C constant current charge ratio = (C4 / C charge) × 100%; (4) High temperature storage performance: At 60°C, charge to 3.65V with 1C constant current and constant voltage, measure the initial thickness of the lithium-ion battery at this time, and then store at 60°C for 30 days to test the thickness of the lithium-ion battery; Wherein, expansion rate (%) = (thickness after storage - initial thickness) / initial thickness × 100%; The lithium-ion battery after high-temperature storage was discharged to 2.5 V at 1C, and the battery capacity retention rate was measured and calculated. The calculation formula is as follows: Capacity retention rate (%) = retained capacity / initial capacity × 100%.

[0059] The test results are shown in Table 2. Table 2

[0060] Compared with Examples 1-8, the difference is that only the content of the second additive is changed. Combined with Table 1-2, it can be seen that the second additive with an appropriate mass percentage can help reduce the problem of increased acidity caused by the introduction of the first additive; moreover, the second additive is oxidized and decomposed in advance before the solvent, forming a stable positive electrode-electrolyte interface film. At the same time, the second additive can also copolymerize with the first additive to form a film, together forming the skeleton of the double-layer interface film, thereby improving the stability of the interface film, avoiding continuous contact between the electrode material and the electrode liquid, effectively preventing the decomposition of the electrolyte while reducing the damage to the electrode material structure. However, the content of the second additive should not be too large, otherwise it will further deteriorate the dispersion uniformity of the second additive inside the electrode under low liquid retention conditions. Therefore, the mass percentage of the second additive should be 0.5% to 2%.

[0061] Comparing Example 2 with Comparative Examples 1-5, Comparative Examples 1-2 did not add the second additive, Comparative Examples 3-4 did not add the first additive, and Comparative Example 5 did not add both the first additive and the second additive. In contrast, Example 2 used a compound of the first additive and the second additive. Combining Tables 1-2, it can be seen that the cycle performance, fast charging performance, and high-temperature storage performance of Example 2 were significantly improved. This is because the second additive can be preferentially decomposed before the solvent in the electrolyte is oxidatively decomposed to form a stable positive electrode-electrolyte interface film; at the same time, the second additive is an ionic liquid additive containing multiple functional groups, and its introduction can significantly increase the number of ion migrations in the electrolyte, so that the electrolyte has a higher conductivity, which helps to increase the ion transfer rate of the electrolyte and improve the fast charging performance of the battery cell. In addition, the amino cationic group of the second additive can stabilize the lithium salt anion, ensuring that the lithium salt anion is stable and not easily decomposed during the cycle storage process. The first additive is introduced into the second additive. Since the first additive has a strong diffusivity and is easily decomposed into a highly electronegative sulfate group when forming a film at the positive or negative electrode, the sulfate group can attack the caged anion group in the second additive, causing its phosphorus-oxygen bond to break. The two act to form a uniform and stable small molecule structure, which can be evenly diffused throughout the entire electrode, solving the problem of uneven distribution caused by limited electrolyte transmission; and the decomposition products of the first additive and the second additive include inorganic salt components such as lithium alkyl sulfonate, lithium phosphate and LiF. The inorganic salt component can improve the stability of the electrolyte and electrode interface at a low liquid retention coefficient. Therefore, Example 2 uses a compound of the first additive and the second additive, which helps to have excellent cycle performance and fast charging performance at a low liquid retention coefficient.

[0062] Compared with Example 2 and Examples 9-15, the difference is that only the content of the first additive is changed. Compared with Table 1-2, as the content of the first additive increases, it helps to provide an appropriate amount of sulfate groups, which is more conducive to attacking the caged anion groups of the second additive. The interaction between the two is conducive to forming a stable small molecule structure, which can diffuse more evenly throughout the entire electrode, thereby being more conducive to improving the interface stability of the electrode.

[0063] Combining Examples 16-18 with Examples 10-12 and Tables 1-2, it can be seen that compared with Examples 10-12, the content of the second additive in Examples 16-18 is increased to 1%. Combining Table 2, it can be seen that the battery has excellent cycle performance and fast charging performance, and the storage stability at high temperature is also relatively excellent. It can be seen that the mass ratio of the first additive to the second additive is in the range of 0.12% to 9%, which can ensure the cycle and fast charging performance of the battery, and has little to do with the total amount of the additives.

[0064] Combining Example 2 with Examples 19-21 and Table 1-2, it can be seen that the second additive in Example 19 contains an alkynyl group. The electron cloud of the alkynyl group is denser and interacts more strongly with lithium ions, which will affect the migration rate of the ions and the cycle performance of the battery. In addition, the alkynyl group is more rigid, which can further improve the density of the film formation and also affect the fast charging performance of the battery. In the second additive in Example 20, R1, R2 and R3 of the amino cation part are selected from C5 cycloalkyl groups. The C5 cycloalkyl group has a certain steric hindrance, which is beneficial to improve the structural stability of the amino cation and enhance the thermal stability of the electrolyte; however, the C5 cycloalkyl group may increase the density of the electron cloud distribution, affect the conductivity of the lithium ions, and thus reduce the cycle performance of the battery.

[0065] Comparing Example 2 with Examples 22-23, Examples 22-23 replaced the positive electrode active material of Example 2. Combined with Tables 1 and 2, it can be seen that under the same injection volume conditions, the capacity retention rate and fast charging performance of the battery are not much different from those of Example 2, but there is a slight impact on the cycle performance.

[0066] Compared with Example 2 and Example 24-25, Example 24-25 further reduces the injection amount. Combined with Tables 1 and 2, it can be seen that the battery's liquid retention efficiency, capacity retention rate, fast charging performance and cycle performance will be affected.

[0067] The embodiments of the present invention are described in detail above. Specific examples are used herein to illustrate the principles and implementation methods of the present invention. The description of the above embodiments is only used to help understand the method of the present invention and its core ideas. At the same time, for those skilled in the art, according to the ideas of the present invention, there may be changes in the specific implementation methods and application scopes. In summary, the contents of this specification should not be understood as limiting the present invention.

Claims

1. An electrolyte, characterized in that: The electrolyte includes a first additive and a second additive; The first additive has a molecular structure as shown in Formula I: Formula I; The second additive has a molecular structure as shown in Formula II: Formula II; in, R1, R2 and R3 are each independently selected from halogen atoms, C1-C 10 Chain alkyl, C3-C 10 Any one of the cycloalkyl groups; R4 is independently selected from any one of C2-C5 alkenyl and C2-C5 alkynyl; Dashed lines represent conjugated π bonds.

2. The electrolyte according to claim 1, characterized in that The second additive is selected from at least one of the following compounds: Formula II-1; Formula II-2; Formula II-3; Formula II-4.

3. The electrolyte according to claim 2, characterized in that The mass ratio of the first additive to the second additive is 1:(1-4).

4. The electrolyte according to claim 2, characterized in that Taking the total mass of the electrolyte as a benchmark, The mass percentage of the first additive is greater than or equal to 0.1% and less than or equal to 4%; and / or The mass percentage of the second additive is greater than or equal to 0.1% and less than or equal to 4%.

5. The electrolyte according to claim 4, characterized in that The mass percentage content of the first additive is greater than or equal to 0.2% and less than or equal to 2%; and / or The mass percentage of the second additive is greater than or equal to 0.2% and less than or equal to 2%.

6. The electrolyte according to claim 4, characterized in that The electrolyte injection coefficient is 2.9 g / Ah to 3.2 g / Ah.

7. The electrolyte according to claim 4, characterized in that The electrolyte further includes a lithium salt, and the lithium salt includes at least one of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, lithium difluorooxalatoborate, lithium bis(trifluoromethylsulfonyl)imide, and lithium bis(oxalatoborate).

8. The electrolyte according to claim 7, characterized in that Based on the total mass of the electrolyte, the mass percentage of the lithium salt is 10% to 18%.

9. The electrolyte according to claim 1, characterized in that The electrolyte also includes a non-aqueous solvent, and the non-aqueous solvent includes at least one of ethylene carbonate, propylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methylpropyl carbonate, methyl formate, ethyl acetate, methyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, propyl propionate, butyl propionate, methyl butyrate, ethyl butyrate, and ethylene glycol dimethyl ether.

10. A lithium ion battery, characterized in that: The lithium-ion battery comprises a positive electrode sheet, a negative electrode sheet, a separator and the electrolyte according to any one of claims 1 to 9; Wherein, the positive electrode sheet includes positive electrode active materials, and the positive electrode active materials include LiCoO2, LiMn2O4, LiMnO2, Li2MnO4, LiFePO4, Li 1+a Mn 1-x M x O2、LiCo 1-x M x O2、LiFe 1-x M x PO4 and Li2Mn 1-x M x At least one of O4; wherein, M is selected from at least one of Ni, Co, Mn, Al, Cr, Mg, Zr, Mo, V and Ti, 0≤a<0.2, 0≤x<1.

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